This invention relates generally to an improved ground plane design of simplified feedthrough terminal pin subassemblies and related methods of construction, particularly of the type used in implantable medical devices such as cardiac pacemakers and the like, to decouple and shield undesirable electromagnetic interference (EMI) signals from the device. More specifically, this invention relates to a reduced cost and reduced mechanical stress hermetic feedthrough terminal pin and ceramic feedthrough capacitor assembly including one or more filter capacitors, and related installation method. It is adapted particularly for use in connecting a lead wire or electrode through a hermetically sealed housing to internal electronic components of the medical device while decoupling EMI against entry into the sealed housing. This invention is specifically designed for use in cardiac pacemakers (bradycardia devices), cardioverter defibrillators (tachycardia devices) and combined pacemaker defibrillator devices. This invention is also applicable to a wide range of other EMI filter applications, such as military or space electronic modules where it is desirable to preclude the entry of EMI into a sealed housing containing sensitive electronic circuitry.
Feedthrough terminal pin assemblies are generally well known in the art for connecting electrical signals through the housing or case of an electronic instrument. For example, in implantable medical devices such as cardiac pacemakers, defibrillators or the like, the terminal pin assembly comprises one or more conductive terminal pins supported by an insulator structure for feedthrough passage from the exterior to the interior of the medical device. Many different insulator structures and related mounting methods are known in the art for use in medical devices wherein the insulator structure also provides a hermetic seal to prevent entry of body fluids into the housing of the medical device. Said hermetic insulators for medical implant applications are typically constructed of alumina ceramic or glass wherein the terminal pins and ferrule are of suitable biocompatible material such as platinum, platinum-iridium, niobium, or tantalum and titanium respectively. However, the feedthrough terminal pins are typically connected to one or more exterior lead wires, for example, the leads which connect a cardiac pacemaker to the ventricle chamber of the heart, which also effectively act as an antenna and thus tend to collect stray EMI signals for transmission into the interior of the medical device. In many prior art devices, the hermetic terminal pin assembly has been combined directly with a ceramic feedthrough filter capacitor to decouple interference signals to the housing of the medical device. A major market force within the medical implantable device industry has been to reduce cost of the implanted device (e.g. pacemaker or implantable cardioverter defibrillator). Medical insurance carriers, government healthcare programs (e.g. Medicare) and health maintenance organizations (HMOs) are placing additional competitive pressures on the manufacturers of such devices.
In a typical unipolar construction, as described in U.S. Pat. No. 5,333,095 (the contents of which are incorporated herein), a coaxial ceramic feedthrough filter capacitor used in a feedthrough assembly to suppress and decouple undesired interference or noise transmission along a terminal pin comprises a so-called discoidal capacitor having two sets of electrode plates embedded in spaced relation within an insulative dielectric substrate or base, formed typically as a ceramic monolithic structure. One set of the electrode plates is electrically connected at an inner diameter cylindrical surface of the discoidal capacitor structure to the conductive terminal pin utilized to pass the desired electrical signal or signals. The other or second set of electrode plates is coupled at an outer diameter surface of the discoidal capacitor to a cylindrical ferrule of conductive material, wherein the ferrule is electrically connected in turn to the conductive housing of the electronic device. The number and dielectric thickness spacing of the electrode plate sets varies in accordance with the capacitance value and the voltage rating of the discoidal capacitor. In operation, the discoidal capacitor permits passage of relatively low frequency electrical signals along the terminal pin, while shielding and decoupling/attenuating undesired interference signals of typically high frequency to the conductive housing. Feedthrough capacitors of this general type are available in unipolar (one), bipolar (two), tripolar (three), quadpolar (four), pentapolar (five), hexpolar (six) and additional lead configurations. The feedthrough capacitors of this general type are commonly employed in implantable cardiac pacemakers and defibrillators and the like, wherein the pacemaker housing is constructed from a biocompatible metal, such as titanium alloy, which is electrically coupled to the feedthrough filter capacitor. As a result, the filter capacitor and terminal pin assembly prevents entrance of interference signals to the interior of the pacemaker housing, wherein such interference signals could otherwise adversely affect the desired cardiac pacing or defibrillation function.
In the past, feedthrough filter capacitors for cardiac pacemakers and the like have typically been constructed by preassembly of the discoidal capacitor within a cylindrical terminal pin subassembly which includes the conductive pin and ferrule. More specifically, the terminal pin subassembly is prefabricated to include one or more conductive terminal pins supported within the conductive ferrule by means of a hermetically sealed insulator ring or bead. See, for example, the subassemblies disclosed in U.S. Pat. Nos. 3,920,888; 4,152,540; 4,421,947; and 4,424,551. An improved design which has substantially improved the volumetric efficiency is based upon surface mounting of a ceramic feedthrough capacitor planar array structure to one outer surface of a hermetic terminal with similar connection to the conductive pins (see the subassemblies disclosed in U.S. Pat. No. 5,333,095). In all of the prior art described above, the outer feedthrough capacitor electrode plate sets are coupled in parallel together by a metallized layer which is either fired, sputtered or plated onto the ceramic capacitor. This metallized band, in turn, is coupled to the ferrule by conductive adhesive, soldering, brazing, or the like.
Although feedthrough filter capacitor assemblies of the type described above have performed in a generally satisfactory manner, the manufacture and installation of such filter capacitor assemblies has been relatively time consuming and therefore costly. For example, installation of the discoidal capacitor into the small annular space described by U.S. Pat. No. 4,424,551 between the terminal pin and ferrule can be a difficult and complex multi-step procedure to ensure formation of reliable, high quality electrical connections. The method taught by U.S. Pat. No. 4,424,551 (the contents of which are incorporated herein), teaches the injection of fluidic thermosetting conductive particles into first and second annular cavities (usually by centrifuge operations). As a consequence, this method also requires insulation of the interface between the capacitor structure and insulator, curing of the various thermosetting materials, and subsequent cleaning operations to remove excess conductive material. While the method taught by U.S. Pat. No. 5,333,095 is far simpler, a connection from the capacitor outside diameter and the conductive ferrule is still required.
A high integrity hermetic seal for medical implant applications is very critical to prevent the ingress of body fluids into the implanted device (e.g. pacemaker). Even a small leak rate of such body fluid penetration can, over a period of many years, build up and damage sensitive internal electronic components. This can cause catastrophic failure of the implanted device. The hermetic seal for medical implant (as well as space and military) applications is typically constructed of highly stable alumina ceramic or glass materials with very low bulk permeability. A helium fine leak test is typically used in conjunction with a sensitive detector to reject defective or cracked hermetic seals. This final product quality conformance test is typically of very short duration (a few seconds helium exposure). This short test exposure will readily detect a leak in a cracked or otherwise defective alumina ceramic or glass hermetic seal; however, it typically takes much longer for helium to penetrate through an epoxy or polymide adjunct barrier (such polymer overcoating can mask the leak).
Withstanding the high temperature and thermal stresses associated with the welding of a hermetically sealed terminal with a premounted ceramic feedthrough capacitor is very difficult to achieve with the ""551, ""095 and other prior art designs. The electrical/mechanical connection to the outside perimeter or outside diameter of the feedthrough capacitor has a very high thermal conductivity as compared to air. The hermetic bonding operation typically employed in the medical implant industry to install the filtered hermetic terminal into the implantable device housing typically involves a laser welding operation in very close proximity to this electrical/mechanical connection area. Accordingly, in the prior art, the ceramic feedthrough capacitor is subjected to a dramatic temperature rise. This temperature rise produces mechanical stress in the capacitor due to the mismatch in thermal coefficients of expansion of the surrounding materials. In addition, the capacitor lead connections in the prior art must be of very high temperature materials to withstand the high peak temperatures reached during the welding operation (as much as 500xc2x0 C.). A similar situation is applicable in military, space and commercial applications where similar prior art devices are soldered instead of welded by the user into a bulkhead or substrate. Many of these prior art devices employ a soldered connection to the outside perimeter or outside diameter of the feedthrough capacitor. Excessive installation soldering heat has been known to damage such devices.
Accordingly, there is a need for a novel feedthrough filter capacitor assembly that addresses the drawbacks noted above in connection with the prior art. In particular, a novel capacitor assembly is needed that is subjected to far less temperature rise during the manufacture thereof by eliminating an outside perimeter or outside diameter electrical/mechanical connection. Such a design would allow the use of much lower temperature materials (such as standard solder) to achieve the capacitor inside diameter lead connections. Moreover, such an improvement would make the assembly relatively immune to the aforementioned stressful installation techniques. A novel filter capacitor design is needed which is of simplified construction, utilizing a straightforward and uncomplicated feedthrough terminal pin subassembly that can result in manufacturing cost reductions. Of course the new design must be capable of effectively filtering out undesirable electromagnetic interference (EMI) signals from the target device. The present invention fulfills these needs and provides other related advantages.
The present invention resides in improved ground plane designs for an internally grounded ceramic feedthrough filter capacitor assembly for shielding and decoupling of a conductive terminal pin or lead of the type used, for example, in an implantable medical device such as a cardiac pacemaker, cardioverter defibrillator, cochlear implant, neurostimulator or the like to prevent the passage of externally generated electromagnetic (EM) fields such as interference signals caused by digital cellular telephones. The feedthrough filter capacitor assembly is typically mounted upon a conductive substrate such as, for example, a conductive pacemaker housing. The assembly comprises, generally, at least one conductive terminal pin and means for mounting the terminal pin for passage through an opening formed in the conductive substrate with the terminal pin and the substrate in non-conductive relation. A feedthrough filter capacitor is provided which has first and second sets of electrode plates. The terminal pin extends through a first passageway through the feedthrough filter capacitor in conductive relation with the first set of electrode plates. A ground lead extends into a second passageway through the feedthrough filter capacitor and is conductively coupled to the second set of electrode plates and the conductive substrate.
The terminal pin mounting means comprises a conductive ferrule adapted for mounting onto the substrate in a position extending through the substrate opening, and insulator means for supporting the terminal pin from the ferrule in electrically insulated relation. The terminal pin, ferrule and insulator means comprise a prefabricated terminal pin subassembly.
Various embodiments of the invention with an improved ground plane design are disclosed herein which illustrate (1) that the feedthrough filter capacitor may be asymmetrical as well as symmetrical about the ground lead, (2) that the ground plane and grounded terminal pin may be machined in one piece as an integral part of the machined ferrule of the hermetic insulator, (3) that the terminal pin may be added by welding or brazing to an machined ground plane which is an integral part of said ferrule, (4) that the ground plane and grounded terminal be co-formed by metal stamping of a ground plate which is subsequently attached to the ferrule by welding, brazing, bonding or the like, (5) that the ground plane be constructed of thin metallic material which is chemically etched to the proper shape which is subsequently attached to the ferrule by welding, brazing, bonding or the like with a ground terminal pin subsequently added by any of the aforementioned means, or (6) that the ground plane be deposited by sputtered or equivalent metal deposition process to the insulator surface with a ground terminal pin subsequently added by any of the above aforementioned means. The sputtered surface may optionally have additional metal added to increase its thickness and thereby increase its effectiveness as an EMI shield and low impedance ground plane. For example, additional thickness may be achieved by plating on a metallic coating of various conductive materials such as nickel, gold or the like.
As described in U.S. Pat. No. 5,905,627 (the contents of which are incorporated herein), any of the improved ground plane designs may include a wire bond pad at one end thereof, and the ground lead may comprise a solid pin or a hollow gas back-fill tubelet, etc. The ground lead may comprise a nail-head lead having one end that abuts a portion of the conductive ferrule and, if desired, the nail head lead may extend from the conductive ferrule through and beyond the feedthrough filter capacitor to provide a ground pin. Alternatively, the ground lead may comprise a ground pin that extends through the conductive ferrule and feedthrough filter capacitor. In this case, means may be provided for hermetically sealing passage of the terminal pin and the ground pin through the conductive substrate. A desirable feature of the improved ground plane design as described herein is that it may be easily added to an existing hermetic seal terminal so that said hermetic seal is thereby retrofitted to accept the internally grounded feedthrough capacitor. Also described is a method to form a hermetic seal and ground pin directly to the titanium housing or subplate of an implantable medical device or the like thereby eliminating the need for a separate and expensive ferrule and the subsequent installation process wherein said ferrule is hermetically and conductively attached to said housing, for example, by laser welding.
Utilization of an internally grounded feedthrough filter capacitor as disclosed herein permits use of a capacitor having non-metallized (insulative) exterior surfaces. This is of particular importance in miniature high voltage feedthrough capacitor devices of the type, for example, used in implantable defibrillators wherein the gap distance between each high voltage terminal pin and ground must be as large as possible to prevent HV breakdown or flashover across the capacitor surface. Elimination of the metallized ground band around the outside diameter or perimeter of the feedthrough capacitor effectively increases the HV flashover distance by the height of the capacitor itself. A ferrite bead disc inductor may also be utilized in connection with the feedthrough filter capacitor to enhance the filtering performance and characteristics of the capacitor assembly.
In one preferred form of the invention for medical implant applications, the feedthrough filter capacitor assembly includes a terminal pin subassembly having at least one terminal pin supported within a conductive ferrule by a hermetically sealed insulator structure. The ferrule is adapted for mounting into a conductive pacemaker housing by welding or brazing to support the terminal pin subassembly for feedthrough passage to the interior of the housing. A ceramic feedthrough capacitor is mounted at either an inboard side or on the body fluid side of the terminal pin subassembly, with capacitor electrode plate sets coupled respectively to a ground pin and to active terminal pin(s) by conductive adhesive, soldering, brazing or the like. As shown, multiple feedthrough filter capacitors are provided in a substantially coplanar array within a common base structure, with each capacitor in association with a respective terminal pin.
The internally grounded monolithic feedthrough filter capacitors utilized in the assemblies of the present invention advantageously eliminate the need to conductively couple a metallized exterior surface of the capacitor to a portion of the conductive substrate or ferrule, as was required in the prior art.
An important feature of the improved ground plane design(s) as described herein is that it acts as a continuous part of the overall electromagnetic shield or housing of the electronic device to be protected. Accordingly, the improved ground plane with its grounded terminal pin is designed to present a low impedance path between the ground plane electrode set of the feedthrough capacitor EMI filter and said electromagnetic shield/housing. The improved ground plate designs presented herein are particularly efficient in presenting a low impedance path by minimizing the inductive and resistive components in the conductive path between said ground plane electrode set and the electromagnetic shield housing. More particularly, the radial shape of the improved ground plane acts to reduce inductance between the electromagnetic shield housing (or ferrule of the insulator terminal) and the grounded terminal pin which connects to the feedthrough capacitor ground plane electrode set. Resistive components are minimized by the use of high quality electrical connections such as co-machining, welding, brazing, conductive thermosetting polymers, solder or the like between the electromagnetic shield/housing and the ferrule, the ferrule and the ground plane, the ground plane and the grounded terminal pin and between the grounded terminal pin and the feedthrough capacitor ground plane electrode plate set.